This is SCIENCE FRIDAY. I'm Ira Flatow. Imagine stepping onto the Earth two billion years ago, taking a stroll along the shores of an ancient beach near the northern edge of what today is Lake Superior. You wouldn't see any trees. They didn't hit the scene until, oh, another billion-and-a-half years. What you might see, though, if you had a microscope, were tiny bacteria-like organisms on the shore having a ball eating each other.

But it wasn't so much of what you might see in our imaginary stroll. What we're going to be talking about, it's what you might smell, because these microbes may have perfumed the shores of this ancient lake with a recognizable scent of rotten eggs. Mmm. That's according to a paper out this week in the journal Proceedings of the National Academy of Scientists.

My next guest was one of the authors. Martin Brasier is a professor of palaeobiology at the University of Oxford in England. He joins us by phone. Welcome to SCIENCE FRIDAY.

MARTIN BRASIER: Hello, nice to be with you.

FLATOW: You believe that at one time, say, a few billion years ago, the whole Earth may have smelled like rotten eggs?

BRASIER: No, that's not what we really would be saying in the paper. We were saying that this particular area, there was this particular gas, which we associate with the smell of rotten eggs, hydrogen sulfide, was sufficiently prevalent that it helped to preserve, in fact, some of the earlier signs of fossilized life in these rocks.

So it wouldn't have been everywhere, but it would probably have been a little bit more noticeable than it is now.

FLATOW: Tell us about the rocks you were looking at. What was inside of them?

BRASIER: Yeah, well, the locality we were looking at is historically rather interesting. It's on the northern short of Lake Superior and it was almost exactly 60 years ago some of the oldest fossils were ever found in it by a Harvard professor, Elso Barghoorn, and an American mineralogist, Stanley Tyler.

And they noticed that inside some very glassy rocks from this particular locality, you could see little filaments, tiny little filaments about a thousandth or so of a millimeter across, preserved inside and that they had the appearance of what we would now call blue-green algal cyanobacteria. Well, this signal has been known for quite a while, and there are a range of different types, but we recently re-examined the material and discovered that funny things were happening to this particular assemblage so that the algal remains were being consumed in some way.

Some of them were full of holes, some of them much more full of holes than other types, and that in fact there was an association between these patterns and they're being turned into a material called iron sulfide or pyrite. People may know it as fool's good.

And this is the product of hydrogen sulfide. It means that there was actually a gas, H2S, smelling like rotten eggs, being produced as the waste product by creatures eating other creatures.

FLATOW: So when you find pyrite, you're finding the waste product of the creatures?

BRASIER: Very often that's the case, certainly in surface sediments if they come from a lake or from the sea. They're the waste product. We can test them by looking at various isotopes. So there are different isotopes of sulfur, and there's a specific pattern of the isotopes of sulfur that's characteristic for this particular pathway.

And we were able to find that, and not only that but in some examples of the material we looked at we could see little sort of bean-shaped cells, not much more, again, than a micron or a thousandth of a millimeter across, which were sitting on the tissue on the cells of material that was itself being consumed.

So we hypothesize anyway that these may be the creatures caught in the act of eating other creatures.

FLATOW: And so the world was filled at that time with these microscopic creatures, right?

BRASIER: Yes, these would be very ancient. And we've long suspected that this sort of thing goes back or should go back a long, long way. And people have found the chemical signals, the brassy fool's gold they've found. They've even found the isotopic shifts. But this is, I think, the earliest record where we can see actually turned into this shiny material the fossils themselves. We can see them actually preserved in three dimensions.

And it's new techniques that we have now that we can use to analyze these in three dimensions and start to pick out their relationships. And that's one of the exciting things that's happening at the moment.

FLATOW: Would there be other places around the world where you'd find the same rocks and fossils?

BRASIER: Yes, I think there probably would be. There are certainly places in Australia and potentially in South Africa where one could look. But the number of places where rocks of this age are preserved or well-preserved is rather few. Most of them have got sort of cooked and baked and shoved up and down by mountain buildings and the like.

And so you often have to look inside the interiors of very large and very ancient continents: North America around the margins of Canada; or in Australia, both of which are very ancient continental masses.

FLATOW: So what happened to the smell? Why doesn't the world smell like rotten eggs today?

BRASIER: Well, it's true that these bacteria are still active, and of course if you pass a blocked drain or something like that, you can see that smell, and it's exactly the same activity going on. Bacteria are eating other things, eating algae, eating other bacteria. And the reason you don't smell it is because the creatures that were living at the surface, the fossils that we first saw, were producing oxygen by photosynthesis.

And that oxygen has now built up to such a level that it destroys the hydrogen sulfide. So it doesn't normally get up into the atmosphere. You don't notice it very much. So that's probably the reason, really - there's too much oxygen, or it's being consumed by other kinds of bacteria a little bit further up the food chain.

FLATOW: Do we humans have similar bacteria living in our guts? We talk a lot about the human biome a lot on this program.

BRASIER: Yeah, my understanding is that this kind of bacterium is found in the guts of many people and is responsible for sulfurous smells. And it's somewhat unavoidable. They're so widespread that they're practically everywhere, and they're probably in the digestive tract of many of us helping to break down the materials that we've had for dinner. So they're there inside us as well.

FLATOW: So we're actually carrying around remnants of billions of years ago within our bodies?

BRASIER: Yeah, I think it's a rather fascinating thought. I'm rather intrigued to follow up other aspects of this. Many of the creatures that were living at the surface of the planet and helping to make it what it was back then are now living inside us, where they're protected from oxygen. So they're living in a kind of space suit. But there they are. They're still around us and inside us.

FLATOW: We're sort of the zoo that keeps them alive.

BRASIER: Yeah, that's right. We are a kind of zoo of deep prehistory, and not just us, of course, but pretty well all the other living things around us. Anything that's got a digestive tract and is consuming food matter of some sort is likely to have a very rich flora of bacteria and primitive organisms hopping around.

FLATOW: If you can find these kinds of primitive organisms, could you find things like evidence of, like, ancient viruses that might have been around?

BRASIER: Well, that's an interesting question, and people have tried to look. The difference between what we've been finding and the virus is that the virus doesn't have a rigid cell wall to protect it. It just invades other things that have cell walls. So it makes it much harder to see and much harder to fossilize. It's often the cell wall or the protective material around it which goes into the fossil record.

But some of my colleagues have thought they have found viral-like structures, sort of polygonal-shaped structures a bit like modern viruses - very, very tiny, but have been unable to convince themselves and other people that no other explanation can be brought forward for these things. So that's still very much an open question.

But it would be nice if we could, but a little bit hard to really confirm it.

FLATOW: Now, for folks who are getting out, the weather is getting nice, spring is upon us, tell me about the spot where the fossils came from. Could you go out there and still find that same spot as you walk about?

BRASIER: Yes, you can. It's actually on a - there is a trail out towards this site. It's quite near to Thunder Bay, and it is - it's a national park. So obviously it's protected, and collecting is not allowed or anything like that. But you can find the Gunflint Trail. There's a Gunflint Trail in the USA, and there's an equivalent one near to Thunder Bay and near to the town of Schreiber in Ontario.

So it would be possible, but it's a very long hike. It's something like a 10-mile hike from where you leave the car. So you would have to be...

FLATOW: Bring lunch.

BRASIER: You would have to be very enthusiastic and energetic. And I have met hikers there on that locality, but it's - you want very good weather because the rocks are very slippy(ph), and they slope down into Lake Superior, which is not a good piece of water to disappear into.

FLATOW: How did the first geologists find this spot there? Did they stumble on it? Where they looking for it?

BRASIER: It's a very interesting question, and Stanley Tyler was hunting for iron. In fact, the deposits which these come from are the rock type from which much of the iron that built up the Detroit car industry comes from. So he was following that, those layers around, and they thought they would possibly find life. And he was - the story goes that he was on a fishing expedition and he actually moored his boat on the shoreline there and happened to notice some very interesting glassy-looking rocks, which were rather black in color.

And he knew that the black meant that there was organic matter, that it was carbon, a bit like coal, and collected some material, cut it into little slices and then sent it off to his friend in Harvard, who confirmed that there were amazingly beautiful fossils inside.

These were the first really good fossils from that far back in time, and when they were first found, nobody knew they were as old two billion years. They were thought to be a few hundreds of millions of years. But now we know they're very ancient.

FLATOW: Very interesting. Thank you, thank you, Dr. Brasier.

BRASIER: You're very welcome indeed.

FLATOW: Quite interesting. Martin Brasier is professor of palaeobiology, University of Oxford in England, and talking with us about going out looking, go looking for those rock formations, maybe this weekend, out there in Canada or in - well, around the Detroit area. Transcript provided by NPR, Copyright NPR.